As a mold for use in an injection molding machine, what is illustrated in FIG. 19a and FIG. 19b is known, for instance. In this drawing, reference numeral 1 denotes a mold, which is configured as a paired structure provided with a stationary mold member 2 and a movable mold member 3. The stationary mold member 2 and the movable mold member 3 are respectively fixed to the stationary platen and the movable platen of an injection molding machine (not shown), for instance. A movement of the movable platen toward the stationary platen results in a closed state in which the movable mold member 3 is in contact with the stationary mold member 2, and a movement of the movable platen away from the stationary platen results in an open state in which the movable mold member 3 is away from the stationary mold member 2.
In the mold 1, when it is in the closed state, the respective mating faces 2a and 3a of the stationary mold member 2 and the movable mold member 3 are in a state of being tightly contacted to each other to form a cavity (molding space) 1a which is to be filled with molten resin. Further, the mold 1 allows a molding A made of a resin solidified within the cavity 1a to be taken out by opening the mold members.
However, if the molding A has an undercut A1 as shown in FIG. 19a and FIG. 19b, the part of the cavity 1a corresponding to the undercut A1 will have to be formed in the core portion 4a of a slide core 4. Incidentally, the core portion 4a may be configured either integrally with or as a body separable from the slide core 4 to which the body can be attached and from which the body can be detached.
The slide core 4, by being installed in a groove-shaped rail portion 3b in the movable mold member 3, is supported to be able to move toward and away from the cavity 1a. The slide core 4 further has a slanted guide hole 4b into which an angular pin 5 is to be inserted. The angular pin 5, fitted to the stationary mold member 2, is so configured as to obliquely protrude from the mating face 2a. 
In the same drawing, reference numeral 2b denotes a locking block which, when the cavity 1a is filled with highly pressurized resin, prevents the core portion 4a from receding from the cavity 1a; reference numeral 3d, a stopper pin for holding the slide core 4 in a retreated position; and reference numeral 3d, an ejector pin 3d for separating the molding A from the cavity 1a part.
In the mold 1 configured as described above, when it is closed as shown in FIG. 19a, the angular pin 5 is inserted into the guide hole 4b of the slide core 4, which is thereby moved toward the cavity 1a and eventually the core portion 4a constitutes a part of the cavity 1a. Then, after the mold 1 is clamped, the cavity 1a is filled with molten resin and, after the resin has solidified, the mold is opened as shown in FIG. 19b. 
When the mold is opened, the slide core 4 is moved by the angular pin 5 in the direction away from the cavity 1a. As this brings the core portion 4a into a state in which it is removed from the undercut A1 of the molding A, the molding A can be separated from the cavity 1a part and taken out by operating the ejector pin 3d. 
There may be a case in which the slide core 4 is installed movably in the vertical direction. In this case, the slide core 4 in the state in which the mold is open is subject to a downward force by gravity, and to prevent a downward movement of the slide core 4 it is necessary to urge the slide core 4 all the time in the direction away from the cavity 1a by providing the slide core 4 with a spring 3e as shown in FIG. 20.
However, if the slide core 4 is provided with the spring 3e, there will arise a problem that the size of the slide core 4 is enlarged, because a hole 4c for embedding the spring 3e has to be bored in the slide core 4. Moreover, as the groove-shaped rail portion 3b for slidably supporting the slide core 4 is provided directly in the movable mold member 3, the whole movable mold member 3 has to be replaced when the rail portion 3b has been worn out by the sliding movement of the slide core 4, resulting in a corresponding increase in the maintenance cost of the mold 1.